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Discussion Starter #1
Where would you start in the troubleshooting of an illuminated brake light when the car stops fine, the pedal is hard, and no brake fluid is being lost?
 

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Discussion Starter #3
Ha! Yeah, I've covered things that obvious at least. The signal is definitely coming from the proportioning valve, because if the electrical connector is disconnected at the valve, the light goes off.

-R
 

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'80 Fleetwood Coupe, 1994 and 1995 Mercedes 140 Coupe
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Has anything been done with the brakes recently?
The proportioning valve will make the electrical connection when one hydraulic circuit loses pressure relative to the other. This can be caused by rear brakes being underadjusted, air in the system or a defective master cylinder. See if the master cyl is full. If not then start looking for a reason. Sometimes a real rapid kick down on the pedal will reset the proportioning valve. The valve has a small pin that sticks out of one end. If the pin is out enough it will turn the light on. It can be reset by pushing the pin back in. The question is why did it go on in the first place. It isn't easy but the best way to adjust drum brakes is to turn the star wheel until the drum will not turn anymore, then push the ratchet arm out of the way and un adjust the star wheel enough clicks that the drum spins without resistance on used brakes and light resistance on brand new installations.
 

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Discussion Starter #6
Thanks Ape Man, for the helpful post! I do suspect the rear brakes may be involved, simply because these are the only parts of the brake system I have not inspected. I guess it could be the proporting valve itself too. Apparently the brake light went on right before I inherited this car. It also needed a new booster, so I replaced this along with the master cylinder. This of course helped with pedal effort, but did nothing for the light. I inspected the front brakes, and everything appears normal there. I'll try resetting the proportioning valve and see what happens. Otherwise it will be time to check out the rear brakes, which I haven't torn into yet partially because I am sufficiently ignorant about drum brakes that I'm a little afraid of them!

-R
 

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1970 Sedan deVille hardtop
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Drum brakes are pretty simple, just snap a digital pic of the innards before disassembling so you know where all the springs go. Also, a set of drum brake spring pliers is a VERY good investment if you want to take them apart.
As for your light, the prop valve is designed to keep a firm pedal and prevent fluid loss when it trips. If there is a leak, the difference in hydraulic pressure slides a shaft in the valve and blocks off the problem circuit.
After installing the new master, when you bled the brakes, did you get good fluid flow at all four corners?
Also, look under the car at the rear brake circuit for anything that looks like it might have been leaking brake fluid. Might not work if you have a greasy undercarriage, but often you can find the problem area by looking for "wet" spots.
 
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